Hydraulic winches and hoists are frequently used to raise and lower a load. For the sake of expediency, these devices will collectively be referred to as winches hereinafter.
The typical hydraulic winch is driven by a reversible hydraulic motor. The motor is rotated in one direction to raise the load by providing pressurized fluid to a first port in the motor while permitting the other port to drain. Conversely, the motor can be operated in the reverse direction by providing pressurized fluid to the second port, and draining the first port.
A number of features have become standard on hydraulic winches. A hydraulic brake is commonly employed which blocks fluid flow from the side of the winch motor which would permit the load to be lowered. The weight of the load simply pressurizes the fluid in the port to prevent the winch motor from rotating to thus act as a hydraulic brake.
Another common feature in hydraulic winch controls is a hydraulic circuit for operating a mechanical brake on the winch. The mechanical brake is constantly urged into the braking position. This action is usually accomplished by mechanical compression springs. A hydraulic cylinder is provided to release the brake during winch movement by providing pressurized hydraulic fluid to act in the cylinder to release the brake. Thus, failure of the hydraulic control circuit causes the brake to be applied.
As can be readily understood, when a load is to be lowered by a winch, some provision must be made to prevent the winch from running away from the operator and lowering the load in an uncontrolled manner. In the past, over center or counterbalance valves have been employed to control lowering. One example of a counterbalance valve is disclosed in U.S. Pat. No. 4,244,275 to Smilges, issued Jan. 13, 1981. The over center valve controls the rate of hydraulic fluid flow from the port of the motor permitting lowering of the load in response to the maintenance of pressurized fluid in the other port of the motor to prevent the winch from running away.
While hydraulic control systems have been available for controlling hydraulic winches to perform the various desired functions noted previously, a need still exists to refine the control systems to minimize the complexity and physical size of the control system, maximize efficiency and reliability of the control system and minimize both the initial cost and maintenance costs of the system.